You may already well know the Azure Multi Factor (MFA) and Azure Self Service Password Reset (SSPR) features; if not you are missing a very good piece of your Azure AD/Office 365 services.

Well, a new registration experience for Azure MFA and Azure SSPR is currently being in preview to simply the end-user registration experience. With the current experience, end-users had to register twice their details (like phone number or secondary email): once for Azure MFA and once for Azure SSPR.

With the new experience, when they register for one of the service (either MFA or SSPR), their details is automatically shared with the other service.

Hi
this is not a private preview; this has been publicly announced
I had the same error for the past few days and it sorted by itself
You can always try to raise a service request to O365/Azure AD support